This invention relates to scintillation cameras which are commonly called gamma cameras. The invention is concerned with improving the resolution and uniformity of such cameras.
In nuclear medicine, scintillation cameras are used to detect gamma ray or other high energy photons emitted from a body in which a radioisotope has been infused. The photons are emitted in correspondence with the extent to which the isotope is absorbed by the tissue under examination. The emitted photons are absorbed in a crystalline material and a scintillation occurs at the point of absorption. The points of absorption are substantially congruent with the point from which they are emitted, since the photons are directed to the crystal with a collimator. An array of photomultiplier tubes, generally hexagonally arranged, are optically coupled to the crystal so that each tube will produce an output signal whose magnitude depends on its particular geometrical relationship to the event being detected. Each tube has an x and y coordinate. The signals from each tube are supplied to a resistor weighting matrix which enables computing the x and y coordinates of each event. The x and y coordinate signals are used to drive a cathode ray tube display or other type of display such that an intensity change is made or written at the coordinate point in the display which desirably should correspond with the true coordinates of the scintillation event. Conventionally, the energy of each event is summed and subjected to pulse height analysis. If the total energy falls within the window of the analyzer, a z pulse is produced which unblanks the display to write the light spot or produce some other kind of intensity change. A photographic film may be used as an integrator of the large number of points or spots of light appearing on the screen of the display. A substantial number of scintillation events is required to make up the final picture of radioactivity distribution in the body tissue. The foregoing is an outline of the basic and ideal features of the well-known Anger gamma camera system disclosed in U.S. Pat. No. 3,011,057.
Problems encountered by designers of gamma camera systems are to optimize uniformity and resolution. Due to the geometrical relationship between the various photomultiplier (PM) tubes, when a source of radioactivity having uniform distribution is placed close to the crystal disc and a photograph is made of the display, the photograph will show non-uniformity which is characterized by "hot spots" under each PM tube and "cold spots" between the tubes. In other words, a spot or scintillation event occurring between the PM tubes is sensed as being partially shifted under the tubes, causing a decrease in spot density or intensity between the tubes and an apparent increase in intensity under the tubes. One known way of reducing this undesirable effect is to move the PM tubes further from the disc. This, however, is at the expense of the ability of the camera to resolve small details. Hence, if small details are to be resolved and if uniformity or correspondence between the generated and displayed image patterns is to be maintained, the output signals from the PM tubes must be modified or corrected.
One method of obtaining correction with nonelectronic means is illustrated in U.S. Pat. No. 3,774,032 which is assigned to the assignee of the present invention. In this patent, the distribution of scintillations as detected by the PM tubes is altered by placing masks between the crystal and the tubes so that light from certain areas of the crystal cannot go directly to the PM tubes. This reduces the output of the tubes for scintillations occurring directly uner them but it permits light from other areas, that is, from between the tubes to go directly to them. The result is better, but not optimized, resolution and uniformity in the image.
It has been proposed heretofore to achieve the results obtained in the cited patent by use of electronic correction means. Electronic correction is based on recognition that if the input and output signals of the preamplifiers which are coupled to the PM tubes are linearly related, the disproportionality between brightness and distance remains, but if the output is modified so that low level signals corresponding with noise are eliminated and high level signals corresponding with the scintillation event occurring at or near the center of the tube are suppressed, more uniform distribution of the light spots on the display will be accomplished.
It has been demonstrated in U.S. Pat. No. 3,953,735 that if the output of the preamplifiers is properly biased, high amplitude signals can be clipped or suppressed which is equivalent to reducing the gain of the preamplifiers for signals above a predetermined amplitude. In this scheme, the plot of preamplifier input signals versus preamplifier output signals is linear for a first comparatively low level signal range and it has a break point after which gain is reduced for higher level input signals. This produces some improvement in uniformity and resolution but there was still localized "hot" and "cold" spots which appeared randomly throughout the crystal, varying from system to system and depending on the individual characteristics of the components of the system.
A further substantial improvement was made in pending patent application Ser. No. 731,150, filed Oct. 12, 1976 now U.S. Pat. No. 4,071,762, dated Jan. 31, 1978. This case is based on the recognition that more than one change in slope of the input to output transfer characteristics of the preamplifiers can eliminate the small localized hot and cold spots which still existed when known techniques for eliminating them were employed. Thus, two or more selected bias voltages are applied to the output of selected preamplifiers to improve linearity or uniformity and resolution.
Other schemes have been proposed such as in U.S. Pat. No. 3,980,886 which uses diodes to couple signals from a nonlinear summing circuit as a feedbck signal for linearizing. Another method is shown in U.S. Pat. No. 3,908,128 wherein a diode or other nonlinear compensation means is biased by an ac and dc source so that diodes in the preamplifier output circuits to the resistor matrix will not always conduct at the identical input signal amplitude.
The foregoing schemes produced linearity or uniformity and resolution improvements which are substantially adequate if all of the photons have the same energy. However, even if the photons from a given isotope are monoenergetic, variations in the scintillation process and the detection process cause the electric signals from the PM tubes to vary, thereby causing the coordinates of the events in the display to lack correspondence with the true coordinates of the events. Thus, it is common practice to normalize the signals which amounts to dividing the coordinate signals by the sum of the energies of each event. Nevertheless, prior linearizing methods are not adequate when more than one isotope having markedly different nominal photon energies are used nor for isotopes which have several different nominal peak energies. For example, radioactive gallium is now being used more frequently for imaging soft tissue tumors or tumors away from the bone. Gallium has three energy peaks and other isotopes have energy peaks which may range between 60 and 450 kilo electron volts. The isotopes most commonly used heretofore generally had an energy range of about 70 to 160 kilo electron volts. On some occasions, more than one isotope is imaged at the same time which makes the prior linearity correction schemes even more inadequate since they can only cope with a small energy spectrum.